To Marnie- great that your DS likes ixl- traditional drills on the computer.
Singapore math is a highly regarded school and homeschool curriculum for teaching elementary age math.
Dreambox (my son's fav): Math games, teaching concepts. We get it free through DS's school.
BrainPoP: a series of fun, videos on a variety of academic subjects including math. The videos are in cartoon form, the explanations transparent, and there are quizzes at the end. I usually let DS choose what interests him, but if there is something in particular that he needs to learn, BrainPoP can help. We bought the younger and older kid version.
Khan Academy: free videos on virtually anything including math. So far, explanations are clear.
If your son likes ixl there is an app (Math App) on the iPad. We're using it for rote memorization of multiplication tables. It's adaptable to go up to 20 and you can set the time allotted for each question. Right off, I will tell you that my son can mechanically and conceptually do long multiplication, but without knowing the tables cold, he struggled with frustration.
I don't actually use the computer as much as it sounds like- but it can work for visual explanation (fractions seemed to require a lot of visual) and to put some fun into math drills/ math facts.
